Secure configuration key provider

We need to store some keys in a secure fashion without increment the solution complexity. After seeing the solution in #2980072, that I think is ok, I propose another way based in hash_salt as an encryption key. Hash salt is used how a secure key to one time links, hash passwords, etc and can be protected to not be stored in settings.php.

We can see in default settings.php that:

/**
 * Salt for one-time login links, cancel links, form tokens, etc.
 *
 * This variable will be set to a random value by the installer. All one-time
 * login links will be invalidated if the value is changed. Note that if your
 * site is deployed on a cluster of web servers, you must ensure that this
 * variable has the same value on each server.
 *
 * For enhanced security, you may set this variable to the contents of a file
 * outside your document root; you should also ensure that this file is not
 * stored with backups of your database.
 *
 * Example:
 * @code
 *   $settings['hash_salt'] = file_get_contents('/home/example/salt.txt');
 * @endcode
 */
$settings['hash_salt'] = 'aaaaaa';

I think that if hash_salt is compromised, all the system is lost. Then, we can base on it how to secure data, because Drupal has it how as a secure point.